Principles of Redox Flow Batteries

In 1973, first investigations have been undertaken at NASA to examine the feasibility and potential of Redox Flow Batteries for large stationary energystorages [2]. In 1976, a patent describing the characteristics of the NASA developed Redox Flow Battery, comprising two completely solved redox couples (Cr2+/Cr3+ und Fe3+/Fe2+), has been registered by L. H. Thaller.

Since the 1980s, a RFB has been developed by M. Skyllas-Kazacos et al., which utilizes solely dissolved Vanadium in four oxidation stages (V2+/V3+ and V4+/V5+). By this approach, the issue of degradation caused by cross-contamination, which caused the developments of the NASA system to be given up, could be avoided.

Within the last decades, in Australia, Japan, the US, China and Europe RFB’s with different redox couples have been developed. The potential of the All Vanadium Redox Flow Battery for stationary storage of electric energy has been shown since the 1990s in various demonstrational and pilot plants in the range from kW/kWh to MW/MWh [4, 5].

By detaching the cell and the electrolyte storage, RFB offer interesting and cost-effective solutions for energy storages with simple scalability, high endurance and an excellent efficiency.

The relative low energy density of All Vanadium VRBs because of the two liquid redox couples is one of the major disadvantages. Next to this, the corrosiveness of vanadium at oxidation stage 5+ and the according high demands on materials, the diffusion of the redox couples through the membrane, causing some self-discharge and the limited solubility of the V5+‐composition leading to a maximum energy density of 37.5 Wh/kg [1] are other boundaries. After all, the very high acidity of the solvents may harm the environment. The VLRFB offers an approach which is likely to combine the specific advantages of both the VRB and a fuel cell.